Switching power supply can be divided into isolation and non-isolation circuit. In this paper, we will focus on the non-isolation circuit. According to different circuit forms, it can be divided into buck circuit, boost circuit, buck-boost circuit, cuk circuit, sepic circuit and zeta circuit. Here, we mainly analyze the working principle of
DC-DC step down converter circuit. As shown in figure 1, power MOSFET is an adjusting switch element and its turn-on and turn-off are determined by the control circuit. L and C are filter elements. When switching off, the diode VD can keep the output current continuous so that it is known as fly-wheel diode. When the output signal of control circuit switch on VT, the current in filter inductor L will increase gradually and the capacitor C will start to charge. Ignoring conduction drop of MOSFET, its source voltage should be Uin.
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Fig. 1. The main circuit of buck DC-DC converter
According to design of TL494 (single ended buck DC-DC converter) and using continuous current mode, when the switching power supply works on CCM, the turn-on time is DT and Ii uplift amount is [(Uin-Uo) *DT]/L in a period; the turn-off time is (1-D)T and Ii uplift amount is [(Uin-Uo) *DT]/L. In the steady state, the two working principles from the top surface of the non-isolated circuit can make ILequal at the end and begining of each cycle. So the following expression can be obtained.
![](/uploads/allimg/150403/2-1504031611029A.png)
Sothe relationship of duty ratio D, Uin and Uo is as follows:
Fig. 2 is the main waveform of CMM buck type switching power supply.In every period, the charge transferred by ILis the same with the charge got by Io at the same time.
![](/uploads/allimg/150403/2-15040316155RX.png)
Fig.2. TheCMM waveform of continuous current mode
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